Our laboratory has had a long-standing interest in the biology of Adenovirus, Ad vectors and how Ad capsid proteins contribute to gene transduction and immune activation. The current proposal addresses the question of how the immune system recognizes a virus at the earliest stages of the host response to infection. The knowledge gained from the proposed studies will allow us to generate new viral vectors that have "stealth" characteristics, and at the same time, will highlight the mechanisms normally resulting in robust inflammation and immunity against wild type Ad. Stealth vectors will allow safer gene transfer and more prolonged expression of transferred genes. The proposed studies are based on a new hypothesis arising from our preliminary data: That two of the major types of antigen presenting cells (APCs)-dendritic cells (DC) and macrophages (MO)-regulate the nature and magnitude of the host immune response to Ad;that APC's detect the virus through a restricted subset of Ad's potential antigens and epitopes;that the dominant epitopes reside in capsid proteins, yet are different for the two types of APC;that the dominant epitope for DC resides in the penton capsid protein;and that the dominant epitope for MO resides in the fiber capsid protein. We have constructed a new set of Ad vectors mutated in penton and fiber that will allow us to test this hypothesis. The results of these studies will not only hold practical implications for gene transfer applications, but will shed new light on fundamental aspects of the earliest phases of viral recognition by the immune system.